Attempts to equip a vehicle with an ultrasonic sensor, laser radar, infrared radar, picture sensor, electric wave radar or the like have been made in order to prevent collision accidents caused by drivers' carelessness or errors of judgment on expressways. However, the ultrasonic radar is influenced by noise, the laser radar or infrared radar is influenced by weather (rain, fog, or snow) or by dust, and the picture sensor has a disadvantage of complicated processing technology, so that each lack practical use. In contrast, the electric wave radar is not significantly influenced by weather, and therefore frequently used for vessels or airplanes, but actually any satisfactory electric wave radar for a vehicle is not provided yet, since the electric wave radar detects clutter from surrounding objects or a road surface. However, a millimeter-wave radar has advantages as described hereinafter. The millimeter-wave radar is shorter in wavelength compared with a microwave radar, thus making it possible to have the following advantages. A transmitting/receiving antenna thereof can be reduced in size (so that a vehicle can be easily equipped with the antenna); the width of an antenna beam can be narrowed (which makes it easier to avoid clutter from surrounding objects); and the relative velocity of objects can be detected with high precision by Doppler frequency.
More specifically, a vehicle with a millimeter-wave radar is provided with a millimeter-wave transmitting/receiving antenna fixed on the front surface of the vehicle, and detects the distance and the relative velocity between the vehicle and objects by receiving with a receiving antenna a reflected wave of a transmitting wave which is transmitted ahead of the vehicle from a transmitting antenna and reflected by the objects. Incidentally, a pulse method, a two frequency CW method, a FM-CW method and the like are used for data processing. In order to detect more than one object at the same time, a filter bank or frequency analysis is used. A fixing position of the millimeter-wave transmitting/receiving antenna to the vehicle is about 70 centimeters from a ground surface on a front surface of the vehicle.
However, a conventional fixing position of the millimeter-wave transmitting/receiving antenna (about 70 centimeters from the ground surface) is intended for vehicles operating at high speed on expressways where the road surface is paved, but is not intended for vehicles for construction and the like operating on an unpaved road surface. Thus, if the above-described conventional art is used for the vehicles for construction and the like without any change, the following disadvantages occur.
(1) A vehicle operating on an unpaved road surface causes pitching or rolling because of unevenness of the road surface or winding. The influence by rolling can be settled, if a transmitting and a receiving antenna are respectively shaped into almost square, for example. But, the influence by pitching, which cannot be compensated for only by change in an antenna shape, causes the vehicle to detect ground clutter, whereby the vehicle cannot detect objects satisfactorily.
(2) Courses of vehicles operating on roads in construction sites, mines, quarries, and the like are usually fixed. Accordingly, researches on fleet operation by more than one unmanned vehicle have been enthusiastically conducted. In the aforesaid fleet operation, vehicles store course data which was previously obtained by teaching, or course data is given to the vehicles from the outside by various sorts of communication means each time. However, on an unpaved road surface, the slip ratio of wheels, for example, changes significantly, depending on weather (rain or snow), soil quality or the like. As a result, even if data on distance and velocity is obtained by detecting the rotational speed of a wheel or the like, the data can be imprecise. Therefore, fleet operation cannot be achieved only with the course data. This is because even if the course data of "turn to the right after operating a certain distance," for example, is given, "a certain distance" cannot be exactly detected. Needless to say, the position or altitude of a vehicle can be confirmed by GPS, but it is well known that the position or altitude of a vehicle cannot be detected in a tunnel or in the shade of a mountain, since the vehicle is in the shadow relative to a satellite.